The Internate

Inheritable aliases in Ruby

Ruby's method aliases are pretty handy. So is inheritance. It's too bad that the two don't work well together:

class Greeter
  def greeting
    fail NotImplementedError, "#greeting is not implemented"
  end
  alias_method :salutation, :greeting
end

class Employee < Greeter
  def greeting
    "Howdy"
  end
end

employee = Employee.new
employee.greeting # => "Howdy"
employee.salutation # => 
# ~> -:3:in `greeting': #greeting is not implemented (NotImplementedError)
# ~>  from -:16:in `<main>'

Wat? Turns out that alias_method creates an alias that references the original method rather than the overwritten one. Fortunately, the Ruby standard library provides a workaround. By using the Forwardable module and its def_delegator method, we can declare a delegator that forwards any call on to the overwritten method.

require 'forwardable'

class Greeter
  extend Forwardable
  def greeting
    fail NotImplementedError, "#greeting is not implemented"
  end
  def_delegator :self, :greeting, :salutation
end

class Employee < Greeter
  def greeting
    "Howdy"
  end
end

employee = Employee.new
employee.greeting # => "Howdy"
employee.salutation # => "Howdy"

Cool, now we have the alias working with inheritance. The only problem with this approach is maintainence. Let's make it clear why we're using something other than alias_method by defining our own inheritable version.

module Aliases
  def inheritable_alias(new_method, original_method)
    define_method new_method do |*args, &block|
      public_send(original_method, *args, &block)
    end
  end
end

Now we can clean our code up.

class Greeter
  extend Aliases
  def greeting
    fail NotImplementedError, "#greeting is not implemented"
  end
  inheritable_alias :salutation, :greeting
end

class Employee < Greeter
  def greeting
    "Howdy"
  end
end

employee = Employee.new
employee.greeting # => "Howdy"
employee.salutation # => "Howdy"

Now there's clarity around why we're not using alias_method. You can tell just by reading the method name. Rad.


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